1. Field of the Invention
The invention relates generally to a variable valve timing system, and, more specifically, to a variable valve timing system that changes the opening/closing timing of a valve by an amount of change corresponding to the operation amount of an actuator.
2. Description of the Related Art
A variable valve timing (VVT) system that changes the phase (i.e., crank angle), at which an intake valve or an exhaust valve is opened/closed, based on the engine operating state has been used. Such variable valve timing system changes the phase of the intake valve or the exhaust valve by rotating a camshaft, which opens/closes the intake valve or the exhaust valve, relative to, for example, a sprocket. The camshaft is rotated hydraulically or by means of an actuator, for example, an electric motor.
For example, Japanese Patent Application Publication No. 2005-120874 (JP-A-2005-120874) describes a valve timing adjustment device that adjusts the valve timing of a valve provided in an engine using a rotary torque produced by an electric motor. The valve timing adjustment device sets a target amount of change in the rotational speed of the electric motor based on the deviation of the actual phase, which is determined based on the rotational speed of a crankshaft and the rotational speed of a camshaft, from the target phase set based on the operating state of the engine. The target amount of change corresponds to the rate of phase change, and the electricity passing through the electric motor is controlled by a drive circuit that receives a control signal indicating the target amount of change in the rotational speed of the electric motor. Japanese Patent Application Publication No. 2005-120874 (JP-A-2005-120874) describes a configuration in which a control circuit produces a control signal having a frequency proportional to the target rotational speed for an electric motor, and a drive circuit controls the electric power supplied to the electric motor based on the control signal from the control circuit (i.e., the inverter control is executed).
A method for transmitting the information between two circuits is described in Japanese Patent Application Publication No. 08-114168 (JP-A-08-114168). JP-A-08-114168 describes the method for transmitting a signal between a knock sensor for an engine and an ECU. According to the method, on the knock sensor side, a result of knock determination made based on the amount of knocking is encoded and the value of the signal, which contains the determination result and which is transmitted through an output signal line, is changed at TDC and the gate close timing. Then, on the ECU side, the value of the signal transmitted through the output signal line is read at two timings and decoded. Thus, the result of knocking determination is reconstructed. With this configuration, a maximum of four determinations, and not only whether knocking has occurred but also the magnitude of the knocking, if the knocking has occurred, are transmitted by the value of only one signal that is transmitted through the output signal line. Also, on the ECU side, the input signal line is reversed, and the TDC is transmitted to the knock sensor. Thus, not only TDC but also the information concerning cylinders is transmitted through only one input signal line.
Japanese Patent Application Publication No. 02-267344 (JP-A-02-267344) describes a configuration in which a power module that drives multiple actuators using multiple drive circuits based on control signals from an electronic control unit is provided, and the information is transmitted from the power module to the electronic control unit through only one signal line by modulating the result of monitoring of the operating states of the drive circuits.
In the variable valve timing systems described above, multiple control modes for an actuator (for example, an electric motor) are usually prepared in order to control the actuator more efficiently. For example, in addition to the forward mode in which the response of the phase to the phase control is maintained at a sufficient level, the special mode may be prepared to protect devices from malfunctioning. In the special mode, the operation of the actuator is controlled based on, for example, the limit position of the range in which the valve phase can be mechanically changed. The valve phase needs to be controlled to be advanced and delayed. Accordingly, the direction in which the actuator should be operated (for example, the direction in which the electric motor should be rotated) needs to be instructed as well.
Accordingly, in the variable valve timing system, control signals that indicate the operation commands for the actuator (the operation amount, the direction in which the actuator should be operated, the control mode, etc.) need to be transmitted from a control unit that sets the operation commands based on the status of the valve phase control to a drive unit that controls the actuator.
In this case, the manner in which the above-mentioned operation command items are indicated by the control signals needs to be set such that, even when the drive unit falsely recognizes the control signal, the negative influence on the control is minimized. In addition, preferably, one control signal contains many operation commands in order to reduce the number of signal lines required.